Despite recent advances, battery life continues to be a limiting factor in mobile multimedia systems. Significant energy savings can be achieved by adapting systems at run-time to match the execution requirements of different tasks. This paper introduces an on-line dynamic voltage/frequency scaling (DVS) feedback technique that reduces voltage and frequency to match the playback rate. A PI controller adjusts the decoder's speed to keep constant the occupancy of the buffer between the decoder and the display, effectively matching the average decode rate to the display rate without the need for any off-line profiling. MPEG simulation results show that this technique reduces decoder power consumption while providing strong real-time guarantees.